Help : Tachy Voltage

[Sunray]

I have a rev counter plus some LED's as a replacement for the DOP, this works nicely and it will never give me the buzzer of death.

I'm developing a ardiuno circuit to monitor all the sensors and the rev's are one of them.

Now to save me dragging my Digital Scope (need to use a scope as its very transient) into the van and possibly doing it in as its got a 300v max input voltage, does anyone know if the tachy signal from the engine attenuated in any way? Its not obvious from the circuit diagrams I've seen, appears to come straight from the top of the coil which is 300 or more volts and really noisy.

If its not I have to build an attenuation circuit, but its surprising that something as powerful and noisy as that signal would have a 5+ meter wire carrying it?

[AdrianC]

appears to come straight from the top of the coil which is 300 or more volts and really noisy.

Surely it's not coming from the HT side of the coil, though, just from the LT side - so 12v?

[bigherb]

No it's 300-400v when the coil collapses. Some people put a diode in if they are fitting those cheep Tacho's. Or do it the Smiths way and measure the fluctuating current draw on the 12v side.

[Sunray]

No it's 300-400v when the coil collapses. Some people put a diode in if they are fitting those cheep Tacho's. Or do it the Smiths way and measure the fluctuating current draw on the 12v side.

Thing is this signal is present in the dash of my bus, it was a doddle to fit the rev counter. Its that signal in the dash I am interested in because then I don't have to string a new wire from the engine bay to the dash.

its so transient that to measure it requires a scope but its beyond the operating voltages of mine really, I was wondering if anyone already knew it?

[bigherb]

It's the same voltage at the dash.
Best to get yourself an attenuator probe save ruining your scope if you encounter any spikes.

[nutbox]

Hi there,
Just thought I'd stop by and say you arduino sensor circuit sound interesting ,I have be looking in doing something like that for my van to do the gauges like oil press ,temp and the like ,I have ordered a 3.5" tft screen and look at getting a mega board to.
Definitely interested if you do a write up on how you did it.
Cheers dazz

[Sunray]

Currently at the breadboard stage and teaching myself how to use Eagle PCB and get it to produce PCB's. My query stems from buying a freq to voltage generator. The Arduino can do freq on pin 5 but its quite taxing on the device due to it using interrupt for timing.

Having a voltage to represent the freq is a better solution as there are good 10bit a/d converters on the board. The issue I have is that I've rigged a 555 timer as a simple freq generator but I'm not wanting to test generate what the coil produces so even if I create a circuit my van isn't somewhere I can sensibly work. If it was a nice 12v it would be easier. I'm now tempted to buy a cheap rev counter, dismantle it, connect it up and probe it out for varying voltages to revs.

I'm using the Nano simply because its got more than enough inputs for what I want and its tiny.

What has currently stalled me is that the display got while quite cool, has the worst library ever so I was writing a new one and as I do that for a living got a bit bored as its dull stuff. Need to get a can of motivation to get past that part.

[nutbox]

Cool, I have been messing about with a arduino uno and done a few projects with and made a focuser for a telescope with it using a stepper motor but I have never wrote a sketch for it which I must start learning,is it hard to do?
Definitely be interested to see what you come up with,I have had a dig around and I think this thread on arduino cc forum might help you http://arduino.cc/forum/index.php/topic,17977.0.html it looks like using the low tension part of the coil ,or off the alternator might be the way and maybe throw a 5v Zener in the to earth for good measure.
Anyway keep us posted and good luck dude

[Aidan]

IMHO the signal wire from the coil to the bod is 12V I bet, it comes from the LT side, I might even stick a meter on it and check, got an old one I don't mind frying